Syncing Google Mail Via Mac Mail With Zimbra Server

Hi i am in the middle of download 1,600 gig worth of emails for our business to a new imac from out google apps page, i will after the downloading is complete (4/7 days) want to upload them once ive removed our domain with google mail to our onsite server, via imap sync,

question is if i sync will it delete them as there is nothing in the server, or will it then upload the files???

Also as all is on the same network once on the imac is there a quicker way to upload them than using our internet?